Deposition velocities to Sorbus aria, Acer campestre, Populus deltoides × trichocarpa ‘Beaupré’, Pinus nigra and × Cupressocyparis leylandii for coarse, fine and ultra-fine particles in the urban environment
Introduction
Particulates in the air are a serious problem in industrial and urban areas globally and there has recently been renewed interest in the role which trees can play in improving urban air quality (Hewitt, 2003, Powe and Willis, 2004). Derwent et al. (2002) reported summertime formation of fine particles and their effects on human health as one of five major environmental problems that would be significantly improved if the United Nations Economic Commission for Europe NOx protocol was implemented. Particles are either primary; emitted directly from power stations, motor vehicles, cement factories, etc., or secondary, formed in the atmosphere through reactions of other pollutants such as SO2, NOx, ammonia and VOCs (DoE, 1995). In the 1950s and 1960s carbonacious soot from combustion of coal formed a major part of urban pollution. As cleaner fuels have replaced coal, secondary particles such as ammonium sulphate and primary particles emitted from vehicles have become more important. In many cities today, vehicles are a significant source of particles (Janssen et al., 1997). Although emissions are less from more modern, efficient petrol and diesel engines, this benefit is often offset by increased traffic density. Poor air quality, including suspended particles is likely to continue to be a problem in developed countries and to be an increasing problem for countries in transition. Metals such as lead, cadmium, nickel and chromium also occur in atmospheric particulates and many organic species such as poly-nuclear aromatic hydrocarbons (PAH) and dioxins, which are potential carcinogens, are also associated with particles.
Trees and woodlands can be significant sinks for gaseous, particulate, aerosol and rain-borne pollutants (Fowler et al., 1989, Broadmeadow and Freer-Smith, 1996). Pollutant removal from the atmosphere is by wet deposition in rain, snow and mist (particularly windblown aerosols), or by dry deposition. Particles, whether organic species or aqueous and present as aerosols (ammonium sulphate, sulphuric acid, etc.), are deposited via four processes; sedimentation under gravity, diffusion (i.e. by Brownian motion) or by turbulent transfer resulting in impaction and interception. For particles bounce-off can occur so that both the physical and chemical properties of the particle and the absorbing surface influence uptake rates. Size is the main characteristic which determines the behaviour of particles in the atmosphere and it is expressed as aerodynamic diameter (Dp). The size fraction regularly monitored is called PM10 (particulate matter less than 10 μm in Dp). Below PM10 the range of particle sizes found in the atmosphere is usually considered in three groups. Ultra-fine particles which are most commonly formed by the condensation of hot vapours (from incinerators and vehicle exhausts) or by the chemical conversion of gases to particles (e.g. sulphuric acid particles from oxidation of SO2). Such particles have a short lifetime in the atmosphere quickly coagulating into larger particles. Particles in between 0.2 and 2 μm in diameter are more stable in the atmosphere, having a lifetime of 7–30 d because they are not deposited through gravitational settling and scavenging by rain. Most particles greater than 2 μm in diameter are formed by mechanical attrition processes (e.g. soil dust, sea-spray and industrial dusts), and are rapidly deposited by sedimentation. Sedimentation is very effective for ‘large’ particles of . However there is a size range of ca. 0.05–2.0 μm Dp over which no deposition mechanism is very effective (QUARG, 1996).
The size distribution of suspended particulate material has altered over the years as sources changed from being mainly domestic coal burning to vehicles and industrial premises. As well as influencing deposition rates to vegetation, particle size distribution also has a major influence on the risk to human health since smaller particles penetrate further into the respiratory system. The impacts of particles on human health have recently been reviewed by Powe and Willis (2004), but Moolgavkar and Hutchinson (2000) have, for example, identified a positive correlation between hospital admissions from chronic respiratory disease and the index of respirable particles in air in Seattle (USA). Although exacerbation of asthma in children, and bronchial and respiratory problems generally are associated with urban air pollution, clear quantification of the adverse effects on human health remains controversial (Department of Health, 1998, World Health Organisation (WHO), 2000). Diesel pollution is, for example, not thought to cause asthma but it does stiffen the airways turning asthma from an episodic to a chronic condition (Bodey et al., 1999). In the UK standards for particles of aerodynamic diameter less than 10 μm (PM10) have been set at a 24 h daily mean of 50 μg m−3 (AQS, 2000). In the US different values for ambient air quality standards have been set for PM2.5 and PM10 in recognition of the greater risk to human health from smaller particles which are inhaled more deeply into the lungs. The World Health Organisation has adopted a similar approach, providing risk estimates for effects of long-term exposure to PM2.5 and PM10 on human morbidity and mortality. The risks to health are greater for the smaller category of particles (WHO, 2000). A recent study in Britain has estimated that the beneficial effect of woodland as a whole in reducing pollution was to prevent ca. 65–89 deaths which would otherwise have been brought forward and to avoid ca. 45–62 hospital admissions yearly (Powe and Willis, 2004).
As a result of their large leaf areas and the turbulent air movement created by their structure, trees take up more pollution, including PM10 than shorter vegetation (Fowler et al., 1989). There has thus been a long-standing interest in the quantification of dust and PM10 deposition to trees and other vegetation. Table 1 summarises the results of both early and more recent work. Our own interest was initially in particle identification and source apportionment for PM10 captured by the foliage of oaks in a polluted urban woodland (Freer-Smith et al., 1997). We have subsequently looked at the total amounts of PM10 present on foliage of different species at a number of polluted and unpolluted sites (Beckett et al., 2000a, Beckett et al., 2000b). Recently we have measured deposition velocities (Vg) and trapping or capture efficiencies (Cp) of a number of important urban tree species in windtunnels (Beckett et al., 2000c and Freer-Smith et al., 2004). Species specific values of Vg and Cp are required if generic models are to be used to estimate the likely consequences of urban tree planting schemes of different planting design and species composition. Vg values are also available from micrometeorological flux gradient studies (see Table 1) and thus allow comparison of data derived from differing techniques. Furthermore models of pollutant dispersion and transport which run on 10 km, or greater, grid square scale require Vg values. Our own (Beckett et al., 2000c, Freer-Smith et al., 2004) and other windtunnel studies have derived Vg and Cp values for small, pot grown plants based on the interception model developed by Gregory (1973) and Chamberlain (1975).
Our objective in the current study was to derive Vg values for field grown trees of Sorbus aria (Whitebeam), Acer campestre (Field Maple), Populus deltoides × trichocarpa ‘Beaupré’ (Poplar), Pinus nigra (Corsican pine) and × Cupressocyparis leylandii (Leyland cypress) based on field measurements and the Ohm's Law analogy. This approach required particulate uptake values to be measured by the same foliar washing techniques which were used in our earlier field work (e.g. Beckett et al., 2000b), and also for measurement of the particle concentrations in ambient air measured at the same sites using Grim dust monitors. It is known that within a specific size range Vg values increase as particle diameters become smaller (Belot et al., 1994, Quality of Urban Air Review Group (QUARG), 1996), and we therefore separated particle uptake into three categories; PM10—coarse particles with aerodynamic diameter (Dp) <10 μm, fine particles (Dp<2.5 and >0.1 μm) and ultra-fines (). Particle concentrations in air were recorded at two sites for a range of size categories and aggregated into comparable values for uptake of coarse, fine and ultra-fines by tree foliage. Vg values (for specific particle sizes) measured in the field using this approach will then be available for comparison with those measured in wind tunnels or by other field methods (see Table 1). Secondary objectives were, to compare the particulate loadings on conifer foliage in winter and summer, to determine the quantities of insoluble material in the ultra-fine size category and to identify the water-soluble anions and cations present in the ultra-fine size category. There are even fewer data available in the literature for these ultra-fines (aqueous aerosols) than there are for coarse and fine particles. These ultra-fines probably represent the main threat to human health and to date very little is known of the makeup of the fraction of these particles taken up by tree foliage.
Section snippets
Sites and tree plots
The data reported here were collected during a second year of work (1998) at the two sites for which earlier data have been reported by Beckett et al. (2000b). Withdean is a small urban park (15 ha) on the main London road in Brighton, East Sussex. This is a busy road being the main northern route into Brighton and congestion of commuter and public transport vehicles occurs in the morning and afternoon rush hours. The second site was the University of Sussex field site situated in pastureland on
Mass of particulates washed from coniferous foliage in late winter
Fig. 1, Fig. 2 show the mass of coarse, fine and aqueous soluble ultra-fine particles washed from the foliage of P. nigra and × C. leylandii in February 1998. The mass of particulates for each size category was similar on the foliage of both conifers except that P. nigra had significantly (P<0.05) more ultra-fines present on its foliage than × C. leylandii at the Sussex field site (Fig. 2). There were significant effects of tree species with P. nigra having more ultra-fines present (P<0.05) and
Discussion
The four sets of field data presented here provide important new information on the capture of particulate pollutants by trees. It is informative to compare the quantities of the various size categories of particle present on the foliage of the five species used and to look at differences between the two sites; one urban and one rural. The data can further be compared with earlier data from the same sites (Beckett et al., 2000b), with data using similar techniques with other species and sites (
Conclusions
In recent years the deposition of particles to trees has been measured using a variety of techniques in both field studies and wind tunnels (Table 1). Review of these data show that, if compared on an appropriate basis, results are consistent with deposition velocities ranging from 0.02 to 28 cm s−1 (this upper value being at high wind speeds). Uptake of particles is especially effective at large exposure doses and in this study doses were greater in winter and at Withdean in the city than
Acknowledgements
We would like to thank the Biotechnology and Biological Sciences Research Council and the Forestry Commission for funding this research. We would like to thank Nick Jenkins (Air Pollution Officer) and Rob Greenland (Arboriculturist) of Brighton and Hove City Council for their help with this research.
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